What was your reasoning for creating the Blue Morpho Butterfly to have wings that are a solid iridescent blue on top, but have a dark brown underside with spots?

This you have seen intuitively and is the reason for your appreciation of this species and your question here. We did wish to have a beautiful and gentle creature to be a visual delight for human, to make it of very large size with a striking blue iridescent wing surface that mirrored the sky. But then to promote longevity and safety of the creature itself, we designed the wings to have a mottled brown underside so that when at rest and the butterfly folded its wings together vertically, the shiny blue side was not on display but only the brown patterned wing surface, making it look like a withered leaf and very much part of a tree branch. When the butterfly was resting on foliage, this provided a quite effective camouflage to help the butterfly rest safely.